This invention relates to a method for controlling the air-fuel ratio of a mixture being supplied to an internal combustion engine, and more particularly to a method of this kind, which is adapted to effect leaning of the mixture when the engine is operating in a low load region, while maintaining optimal operating characteristics of the engine such as driveability, emission characteristics, and fuel consumption.
A fuel supply control system adapted for use with an internal combustion engine for vehicles, particularly a gasoline engine has been proposed e.g. by Japanese Patent Provisional Publication (Kokai) No. 57-137633, which is adapted to determine the valve opening period of a fuel injection device for control of the fuel injection quantity, i.e. the air-fuel ratio of an air-fuel mixture being supplied to the engine, by first determining a basic value of the valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine cooling water temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
On the other hand, it has also conventionally been carried out to lean an air-fuel mixture being supplied to the engine so as to make the air-fuel ratio of the mixture leaner than a theoretical mixture ratio, to thereby enhance the combustion efficiency of the engine and accordingly save the fuel consumption.
However, there are the following problems in carrying out such leaning of the mixture: First, a three-way catalyst, which is conventionally employed to purify ingredients HC, CO, NOx in exhaust gases emitted from the engine, shows a maximum conversion efficiciency of such ingredients when the air-fuel ratio of the mixture has a value equal to a theoretical mixture ratio. Therefore, in an engine having such a three-way catalyst arranged in the exhaust pipe, it is generally employed to control the air-fuel ratio of the mixture to the theoretical mixture ratio in a feedback manner responsive to the output of an O.sub.2 sensor arranged in the exhaust system of the engine. However, if this feedback control based upon the output of the exhaust gas sensor is carried out when the engine is operating in a mixture-leaning operating region where the air/fuel ratio of the mixture is controlled to a value leaner from the theoretical mixture ratio, the conversion efficiency of the three-way catalyst drops. Further, if such mixture-leaning operation is carried out in an operating region of the engine where the nitrogen oxides NOx are produced in large amounts, it can result in spoilage of the emission characteristics. Furthermore, leaning of the mixture causes a drop in the engine output, which is disadvantageous when the engine is operating in an operating condition requiring large output torque, such as at sudden acceleration and wide-open-throttle operation, wherein leaning of the mixture will cause degradation of the driveability.
In order to avoid the possibility of spoilage of the emission characteristics and driveability of the engine caused by leaning of the mixture which is intended to curtail the fuel consumption, it has been proposed by Japanese Patent Provisional Publication (Kokai) No. 54-1724 to operate an air-fuel ratio control system in closed loop mode to carry out feedback control of the air-fuel ratio of the mixture so as to achieve a theoretical mixture ratio when the engine rotation speed as assumed to correspond to the vehicle speed is within a predetermined range, while operating the same system in open loop mode to set the air-fuel mixture to a value leaner than the theoretical mixture ratio when the engine rotational speed is outside the above predetermined range.
However, since this proposed method relies only upon either vehicle speed or the engine rotational speed for selecting the closed loop mode control or the open loop mode control to control the air-fuel ratio, it will be impossible to achieve all satisfactory operating characteristics of the engine including fuel consumption, emission characteristics and driveability at the same time.
The operating conditions of an internal combustion engine can be divided in a plurality of different operating regions defined by values of engine operation parameters such as engine rotational speed and intake pipe pressure, and it is therefore necessary to control the air-fuel ratio of the mixture to respective different suitable values in such different operating regions. Furthermore, the range of such different operating regions in which leaning of the mixture can be effected has to be varied depending upon the vehicle speed and the engine temperature.